Classification of Valves Used in Piping Systems

Valves exceed any other piping component. Estimates reveal that a substantial portion,

approximately 8-10%, of the total capital expenditure of the chemical process industry is

used for the procurement of valves. In terms of the number of units also. Hence,

proper thought should be given for the selection of valves. The first step in the selection is

to determine exactly what function the valve is expected to perform after it has been

installed.

Valves are installed on equipment/ piping to perform any one of the below mentioned

functions.

Classification of Valves:1. Based on functions:

Functions of Valves

The design of the valves is done in such way as to perform any of the above functions.

The type of valves used can be classified in the following categories.

Isolation valve

Gate Valve

Ball Valve

Plug Valve

Piston Valve

Diaphragm Valve

Butterfly Valve

Pinch ValveRegulation Valve

Globe Valve

Needle Valve

Butterfly Valve

Diaphragm Valve

Piston Valve

Pinch Valve

Non return   valve

Check Valve Special Purpose Valve

Multi-port Valve

Flush Bottom Valve

Float Valve

Foot Valve

Line Blind Valve

Pressure / Vacuum Relief Valve

2. Based on the end connections:

The valves can further be classified based on the end connections. End connection meansthe arrangement or attachment of the valves to the equipment or to the piping. The types ofend connections are:

Screwed ends.

Socket weld ends

Flanged ends

Butt weld ends

Socketted ends

Wafer type ends

3. Based on the materials of construction:

The valves could also be classified based on the materials of construction. There can be

any number of combinations possible with the materials of construction. It is for the

piping engineer to select the same in consultation with the process engineer to suit the

process fluid.

The environment in which the valves are installed is also to be considered for selection

of materials of construction. However, the most commonly available materials are:

Cast Iron

Bronze

Gun metal

Carbon Steel

Stainless Steel

Alloy Carbon Steel

Poly Propylene UHMW-PE,UHMW-HDPE etc

Special Alloys

Body could also be lined with elastomer so that process fluid will not be in contact with

the metal.

The valve could also be classified based on the type of construction. Valve manufacturers

offer endless varieties of construction. Based on the operation, valves can be broadly

classified as operated valves and self-operated valves. Mainly the check valves are self-

operated and all other types come under operated valves.

Isolation Valves: Ball Valve Used in Process Industry

Introduction to Ball Valve- An Isolation Type of Valve

The ball valve is normally used as positive shut off valves. The positive shut off is

attained because of the soft seats. Special design is also available with ball having shaped

port for  regulation. Metal seated ball valves are also available for high temperature service.

They are also considered as quarter turn valves as the closing action is achieved by rotating

the lever by 90Deg.They are one of the most widely used valves in process industries

because of their versatility, support pressures up to 1000 bar and temperatures up to 482°F

(250°C). Sizes typically range from 1/2 to 30 inches. They are easy to repair and operate.

Ball valve Ball valve

The ball valve can be classified based on:

The type of body construction

The port size

The construction of seat

The construction of ball

The above classification is in addition to the ones based on the end connections,

material for construction and the pressure classes. The pressure temperature ratings of

the ball valve is generally established by the materials of the seat rings. The service

temperatures are also limited by the material of seat rings.

The ball valve offers minimum resistance to the flow. There are two types of designs

available as far as the flow area through the valve is concerned. They are the Full Port

design and the Regular Port (Reduced Port) design. In full port valves, the port diameter

will be equivalent to the nominal size of the valve, whereas in the regular port valves, the

port diameter will be one size smaller than the nominal size.

Based on Body Construction Type

Ball valves are also classified based on the body construction :

• Single piece design

• Two piece design

• Three piece design

• The short pattern

• The long pattern

• Sandwich design.

• Flush bottom design

In the single piece design valve, the body will be cast/forged as one piece. The insertion of

the ball will be through the end of the body and is held in position by body insert. This

design restricts the valve to be of regular port only.

In two piece design, the body is constructed in two pieces and the ball is held in position

by body stud. There can be full port or regular port design possible in this construction. In

case of three piece construction, the body has two end pieces and one centre piece. These

are held

by body studs.

The three piece construction permits in line servicing without disturbing the existing pipe

work. If the valves have socket weld, screwed or butt welding ends, this design totally

dispenses with the necessity of companion flanges.

The short pattern and the long pattern of the body is on the basis of the end dimensions

Normally short pattern body is adopted by the manufacturer upto 300 NB valves for 150 LB

class. In case of 300 NB to 400 NB class 150 short pattern valves, the ball may protrude

beyond the body and flange faces when the valve is in closed position.

The sandwich design is the flangeless design adopted by some manufacturers. This is to

confine the use of the high cost exotic materials like Alloy-20. Hastelloy-B. Hastelloy-C etc.

To the wetted areas only. The valve is designed to fit between the flanges. The body cover

gets

bolted to body with studs or hexagonal head screws.

The seat rings are renewable in the ball valves except for those having one-piece sealed

body construction. The two different types of seat construction are possible, viz. The fire

safe design and the non-fire safe design.

In the fire safe design, a secondary metal seat will be provided so that when the soft seat is

fully burnt, the ball will shift its position and seat against secondary metal seat and arrest full

leakage.

In an actual fire, the heat intensity of the fire could be so different that it is impossible to

ensure that elastomer seats are fully damaged during fire. If the seats are only partially

damaged, the ball cannot take seating against the secondary metal seat and hence the

valve

would leak. Hence, in my opinion, none of the soft seated ball valves can be declared fire

safe since the valves are bound to leak in case of partially damaged seats.

The manufacturers have come up with metal seated ball valves which are fully fire safe.

Here the elastomer seats are replaced by metal seats which could even be deposited with

high temperature resistant materials.

The fire safe design should also ensure that any development of static electricity should be

fully discharged by proper design and manufacture of valve. Such an arrangement is called

the ‘Anti-static’ design. This ensures to have a discharge path from ball to the spindle and

from spindle to the valve body with an electrical resistance of not greater then 10 ohms

when the valve is new. A typical method of achieving earthed continuity is to provide

stainless steel spring loaded plungers, one fitted between the stem tongue and ball and

second fitted

between stem and body.

The ball could be of full bore or a reduced bore. The design aspect of the same has been

explained earlier. The ball at the bottom end of the body could be supported fully by the

seat or it could be trunnion supported. The ball can be solid ball or of hollow construction

with

cavity. The cavity is to be sealed when the valves are used in volatile liquid. This design of

the ball is called sealed cavity design.

The gland shall be bolted type or screwed. Internally screwed stuffing box is not allowed by

code. Bellow sealed bonnet is also provided in case of valves used in lethal services. There

are two basic bellow seal designs available. The same is explained under plug valves.

The valves shall be operated by wrench or by hand wheel with gear arrangement. The

wrench shall be designed so that it is parallel to the flow passage of the ball. The valve shall

be closed, by turning the wrench or the hand wheel in a clockwise direction. The length of

the

wrench or the diameter of the hand wheel shall be such that the minimum force is required

to operate the valve under the maximum differential pressure.

When added emission control is required additional packing and leak off port are options

that can be added.

The valve having pressure balanced is called dynamically balanced plug. M/s Audco called

these types of valves as ‘Super H’ pressure balanced valves. The break away torque

required to operate these valves are lower than (almost half) that for the reduced port ball

valves.

Piston Valve: Isolation Valve used in Piping Industry

Piston valve resembles in construction more towards a globe valve and is mainly used

for shut off and regulation.

This valve provides positive shut off. The shut off assembly comprises of a metal

piston, two resilient valve rings and a metal lantern bush. The sealing surface consists

of the outer vertical surface of the piston and the corresponding inner surfaces of the

sealing rings. This provides a large sealing surface compared to globe valves of

conventional design.

Piston valve is of two types, balanced and unbalanced. Balanced valve is used in high

pressure services and unbalanced one for low pressure services.

The main parts of the piston valve can be identified as:

1. Body

2. Bonnet

3. Piston

4. Lantern bush

5. Spindle

6. Sealing rings

7. Yoke bush

8. Indicator

9. Hand wheel

10. Belleville washers

11. Studs nuts

12. Wheel nut

13. Set screw

14. Spindle-Lock nut

The body is normally of cast construction. It can have screwed ends, flanged ends or butt

welding ends. These valves follow the regulatory codes to DIN. There are no API or ANSI

standards covering the piston valves. The end to end dimensions are to DIN 3202 which

is more than a gate or globe valve of the same size to API/ASME/ BS standards. Of late,

the Piston Valves are also made to ASME B16.10 dimensions. The end connections are

also available to ASME/ BS standards.

The bonnet is also of the same material as that of the body and it is of bolted construction.

The piston along with the two resilient seats provides proper sealing. The upper valve

ring seal to atmosphere, the lower valve ring provides seal across the ports.

The lantern ring serves as the distance piece between the two rings.

The valve rings are the heart of piston valves.

The seal rings are made from specially developed high quality elastomer material or

graphite. The materials are selected depending upon the service conditions viz. The fluid in

which the valve is used and its pressure temperature conditions.

Spring washers are fitted under the bonnet nuts to ensure that the pressure of the bonnet

on the valve ring is kept constant. This along with the resilient sealing rings produces a

spring action which compensates for any differential expansion that can occur.

There are two types of stem designs available, the inside screw rising stem and the O.S

and Y type with rising stem. The hand wheel is of rising design. In O,S & Y type, a stuffing

box with a bolted gland is provided. This design is mainly used for Thermic fluid services.

The piston valves are preferred by maintenance people as it needs lesser attention. They

call it as ‘Fit and Forget’ type of valve.

Plug Valve: An Isolation Valve

A plug valve is a quarter-turn on-off valve. The plug can be cylindrical or tapered and

has a variety of port types. Plug valves are available in either a lubricated or non-

lubricated design. Plug valve ends can be flanged, hub type or butt weld.

1. Non-Lubricated Plug Valve

Non-lubricated plug valves contain a polymeric body liner or a sleeve, which is installed in

the body cavity. The tapered and polished plug acts like a wedge and presses the sleeve

against the body. The nonmetallic sleeve reduces the friction between the plug and the

body. Like for soft-seated ball valves, non-lubricated plug valves are limited by temperature

and chemical compatibility of the non-metallic materials used. Non-lubricated plug valves

are used instead of lubricated plug valves, where maintenance needs to be kept to a

minimum. They can be used in special services (e.g. sulphur and hydrogen fluoride), since

they do not have a body cavity in open communication with the flow when switching, where

fouling liquids could be trapped or solidify, potentially jamming the valve. These valves are

not torque seated.

Fig.1: Non-Lubricated Plug Valve

2. Lubricated Plug Valve

Lubricated plug valves will be of inverted pressure balance type only and shall be used if

the product allows the use of lubricant. Lubricants shall contain base oil and a viscosity

improver like amorphous or fumed silica. They perform well in processes where the fluid

carries mildly abrasive particles. The plug and body tapered bores are matched together to

form a relatively wide seat area. The plug bore is normally reduced but full port is available

for slurry applications with high solid content where these valves are particularly more

suitable than other types. Full port valves are considerably more expensive than reduced

port and normally not used in hydrocarbon applications.

The sealant or lubricant establishes a seal between the plug and the body, protects the

seating surfaces against corrosion, and acts as a lubricant, thus reducing the force required

to open or close the valve. The sealant is injected periodically, perhaps after twenty

operations, or after long periods without operation. An automatic lubrication device can be

fitted. Failure to lubricate may render the valve un-operable. In most designs, the plug can

be lifted from the seat through manual adjustment of a jacking screw. Solids may become

trapped between plug and seat after such intervention, impairing reliable shut-off. Damaged

seating can sometimes be restored in service using the sealant facility.

Lubricated plug valves are typically used in dirty upstream applications, where

contamination of the process fluid is not a cause of concern. The valves are however

heavier and more expensive than ball valves. They are also extensively used on gas

pipeline systems as bypass valves to mainline ball valves, blow-down valves on valve

stations and kicker valves. They can be delivered in exotic materials such as duplex SS and

Inconel overlaid.

These valves are not torque seated.

For double block and bleed non-piggable upstream applications, where space and weight is

a consideration, double isolation plug valves may be considered instead of two valves in

series.

Fig.2: Lubricated Plug Valve

Other plug valve designs, which are non-lubricated and can therefore be used downstream,

are the eccentric and expanding plug designs.

3. Eccentric Plug Valve

The eccentric plug valve is essentially a plug valve with the plug cut in half. The advantage

of this design is a higher seating force with minimal friction encountered from the open to

closed position. Shut-off capabilities are improved without a significant increase in operating

torque. These valves are torque seated.

Fig.3: Eccentric Plug Valve

4. Expanding Plug Valve

The expanding plug valve is a special design in which the plug is made of multiple

components allowing it to mechanically expand through a wedging action onto the

cylindrical body. These valves, although expensive, give a true double block and bleed

function in a single valve, and are ideal for preventing product contamination (e.g. in oil

movement operations). However these valves shall not be used as a means of “double

isolation” for maintenance purposes, as it is still likely that a single failure will affect both

blocking functions. These valves are torque seated.

Fig.4: Expanding Plug Valve

Additional design features

Plug valves are double seated valves which incorporate a cavity between the seats, or

above and below the plug; the cavity relief requirements shall be applied.

Non-metal-seated plug valves and non-lubricated plug valves with non-metallic sleeves

handling flammable fluids shall be provided with an antistatic device which achieves

electrical continuity between the plug, stem and the valve body.

Gate Valves Used in Piping Systems

Introduction to Gate Valve

Gate valves (GV) are specifically used in isolation applications in various piping systems.

And they operate in the fully opened or fully closed positions.

Like other isolation  types of valves   like ball valve, plug valve, piston valve, diaphragm

valve, butterfly valve, pinch valve,  gate valves are also isolation valves.

Gate valves are multi-purpose bi-directional shutoff valves for commercial and industrial

applications. The shutoff action is achieved by moving the wedge in vertically up/down

direction in the valve body, because of their ability to cut through liquids.

They are not recommended for regulation purpose as most of the flow change occurs near

shutoff with a relatively high fluid velocity causing disk and seat wear and eventual leakage.

Gate Valves are available with a variety of bonnet types, body and trim materials and stem

packings in addition to a broad range of pressure classes, and end connections including

extended bodies.

Typical Gate valves has various parts which could be identified as

1. Body 10. Valve Port

2. Bonnet 11. Yoke Bush.

3. Wedge 12. Lantern

4. Stem 13. Back Seat Bushing

5. Gland 14. Gland eye bolts & nuts

6. Seat ring 15. Bonnet bolts & nuts

7. Yoke 16. Hand Wheel

8. Packing 17. Hand Wheel nut

9. Gland Flange 18. Bonnet Gasket

Body:

The body in gate valves gets attached to the vessel or piping. The classification of the

body could be done depending on the end connections as indicated earlier. Body could also

be specified based on the material of construction of the same.

The wall thickness and end to end/face to face dimensions of the body shall be as per

the Regulatory code to which it is designed.

The end flanges shall be integrally cast or forged with the body. It can also be attached

by welding, if so specified. The end connection shall suit the rating specified. The

flanged connection shall be to ASME B 16.5 or any of the flange standards.

The butt welding end connection shall be to ASME B 16.25 or any other end preparation

required. The socket weld/screw connection shall be to ASME B 16.11 or any other

equivalent standards. The body can have auxiliary connection such as drains, by-pass

connections, etc.

Bonnet:

The bonnet is classified based on the attachment of the same to the body. The type

of connections normally adopted are Bolted, Bellow sealed, Screwed-on, Welded,

Union, Pressure sealed etc.

The bolted connection shall be flanged, male and female, tongue and groove or ring type

joint.  In low pressure rating valves, it may be flat faced. The bonnet gasket is selected to

suit body bonnet connection. It can be corrugated or flat solid metal, corrugated or flat metal

jacketed, asbestos filled, metal ring joint, spiral wound asbestos filled, flat ring compressed

asbestos for low pressure application, Teflon or Teflon filled for corrosive applications.

The bellow sealed bonnets can be bolted or welded on to the body. These are selected

for very critical services like the nuclear application and very high temperature and

lethal services. The screwed-on bonnet is used for very low priority application and small

sized valves.

Wedge:

This is the part which facilitates the service by its movement up and down. The type

of wedges are classified as

• Solid Plain Wedge

• Solid Flexible Wedge

• Split Wedge

When solid disc is wedged into the rigid body seat and the valve undergoes

temperature changes, the wedge gets jammed in the seat. Hence the flexible wedge and

split wedge design is developed to overcome this difficulty.

Normally the solid plain wedge is referred as solid wedge and the split wedge is referred

as flexible wedge. The design slightly alters with the manufacturers though the basis

remains the same.

The flexible wedge design is followed for valve sizes 50 NB and above. Valves 40 NB

and below are available in solid wedge design only. Flexible wedge design is superior as it

will not get jammed during high temperature operations. The wedge material should be at

least of the same quality as that of the body. In case of integral seat rings the wedge

circumference is deposited with superior quality material. In smaller valves, the whole

wedge will be manufactured out of superior material.

Stem:

The stem connects the hand wheel and the wedge for operations. The design can have

rising stem and non rising stem. The stem is operated rotating the stem nut by hand wheel

mounted at the top of the yoke. In the rising stem design, the stem moves up along with the

wedge to open.

This is called the OS & Y ( Outside Screw and Yoke) type of design. In case of non rising

stem the wedge moves up and down and the stem is stationary. This is called the inside

screw design.

Normally, bar stock or forging are used for the construction of stem.

Gland, Gland flange, Packing and Lantern:

There are two types of gland designs possible, Single piece and Two piece. In two

piece design, there will be gland flange and a follower. The follower will have a spherical

end which facilitates proper aligning of follower and load on the packing. In Single piece, the

gland and follower will be cast integral. This design is used mostly in low pressure valves.

Normally gland follower will be of superior material than the gland flange. Gland flanges

normally are made of carbon steel only. The glands are bolted to the bonnet with gland eye

bolts.

The regulatory codes specify that the stuffing box should accommodate minimum six

packing rings for class 150 valves. As regards higher rating valves, it should have lantern

ring with five packing rings above and two packing rings below lantern. Lantern is not

provided for class 150 valves. Lantern is provided for higher rating if required. When lantern

is provided, the stuffing box shall be provided with two plugged holes. The material of

lantern shall have corrosion resistance equal to that of the body.

Normally, the packing are of braided asbestos with suitable corrosion inhibitor. When

special packing such as ‘Graphoil’ is used, the number of packing rings required will be

more. To accommodate more packing rings, the length of gland is also modified. This

design is called

the ‘Deep Gland’ design. This is used for the high temperature services. But this

cannot satisfy the EPA’s fugitive emission standard of < 500 ppm threshold. Hence frequent

LDASR will result in excessive expenditure.Seat Rings:

There are two types of designs possible in seat rings. They are the integral and renewable.

In case of renewable seat rings, it may be either threaded, rolled in or welded in. In case

of integral seat rings, the seat material is weld deposited directly on to the valve body.

The minimum hardness specified by the code for this material is 250 HB, with 50 HB

minimum differential between body and gate seats, the body seat being harder.

The back seat arrangement is provided to repack the stuffing box when the gate is in

fully open position. The stem shall have an integral conical or spherical backseat surface to

seat against the bonnet backseat.

Yoke and Yoke Bush:

Yoke may be integral with or separate from the bonnet. When the yoke is integral, the

stem nut should be removable without removing bonnet. The yoke should have the same

material of construction as that of the shell. The Yoke bush is normally a Ni resist material.

This is to prevent gauling of the stem as stem will normally be of a Nickel alloy.

Hand wheel and Hand wheel Nut:

The hand wheel is fixed to the stem by a threaded hand wheel nut. The arrow pointing

the direction to open the valve will be marked with the word “open” or “close” or “shut”

unless the size makes it impracticable. Valves shall be closed by turning the hand wheel in

clockwise direction.

The material of construction of hand wheel shall be malleable iron, Carbon steel, Nodular

iron or Ductile iron. Cast iron is not referred. The nut shall be of carbon steel or stainless

steel.

When the installed position of the valve is such that the hand wheel is not accessible, then

the hand wheels are replaced by chain wheels and the valve is operated with chains. For

large diameter valve where the operating torque is high, gear arrangement is provided to

facilitate operation. Mostly, bevel gear equipment is adopted. If remote operation of the

valve is required, then this could be achieved through motor with switches. Proper selection

of the drive unit should be done depending on the services.

Bolting:

Normally high tensile stud bolts are used for bonnet bolts and low carbon bolts for gland

and yoke bolting. Gland bolts are normally hinged bolts with hexagonal nuts.

Valve Port:

The valve size is specified by the size of the end connection or the body end. The port or

the bore is the passage through the valve.

There are two types of port designs possible in gate valves, full bore and reduced bore.

In case of full bore, the net area of the bore through the seat shall be as nearly practicable

equal to the nominal pipe size. For reduced port valves, the port diameter is normally one

size less than the size of the end.

The compact design small bore (1/2 – 1/2 inch) gate valves are as per API 602 or BS

5352. Unless the full bore design is specifically asked for, manufacturers supply the

reduced bore valves. The full bore design gate valves are also covered in BS 5352. In full

bore design, the net area of the bore through seat shall be equivalent to that of Sch 80

pipes for class 800 valves and Sch 160 pipe for class 1500 valves. In no case less than

90% of the above figure is acceptable as per code.